This invention concerns improved means for selectively identifying input tone signals among a plurality of discrete frequency ranges. Such frequency detectors are especially useful for decoding numerical information that has been encoded in terms of multiple voice frequencies for transmission over telephone lines and the like.
For the sake of clarity, the invention will be described illustratively as it relates to the particular multi-frequency code that is typically generated by conventional touch dialing equipment. In that code, as more fully described in U.S. Pat. No. 3,537,001 to Joseph J. Friend, for example, each decimal digit is represented by a pair of simultaneous tones, one selected from an upper group of four harmonically unrelated audio frequencies and the other selected from a lower group of four similar frequencies. An important disadvantage in existing decoders for such touch dialing codes is the difficulty in applying normal multiplexing techniques to them. The same features that make the present invention especially effective for decoding touch dialing signals are also useful in a variety of other frequency detection operations.
As in many previously available systems for decoding such multi-tone codes, the present invention typically first separates the upper and lower tone groups by suitable filters, which may be either analog or digital in nature. The resulting upper and lower signal components are then supplied to distinct multi-tone discriminating circuits, which may be closely similar in design. Each tone discriminator produces an output signal, typically in digital form, representing the received frequency of its group. The two discriminator outputs may be further tested for validity, and are then combined and converted to a suitable code representation of the corresponding decimal digit.
Among the major design considerations in frequency detectors of the described type is the effective combination of a sufficiently rapid response to a valid tone burst and a high degree of discrimination against false digit indications. Since each tone may persist on the line for only about 40 milliseconds (ms), the tone detector has a limited number of tone cycles with which to perform that dual function.
An object of the present invention is to provide circuit means capable of producing especially sensitive and reliable frequency discrimination within a tone burst of limited duration.
Spurious tone identifications can be reduced by making successive determinations of the received frequency, and comparing those determinations for consistency before accepting a result as valid. The larger the number of such tentative determinations, the greater the potential reliability of the final result. But also, in previous systems, the more numerous the determinations, the more hastily each must be completed, and hence the smaller the number of tone cycles upon which it can be based. Thus an increase in reliability due to more determinations tends to be offset by a corresponding reduction in the potential reliability of each. No fully satisfactory solution of that dilemma has been proposed by the prior art.
A further object of the invention is to provide a multi-frequency detector capable of deriving from a limited number of tone cycles a relatively large number of distinct tentative frequency determinations without a corresponding reduction in the number of frequency cycles upon which each determination is based. The significance of a successful consistency test is thereby increased, leading to improved overall reliability of operation.
A further aspect of the invention concerns multiplexing of the frequency detector, whereby a single pair of upper and lower frequency discriminators, for example, can handle a large number of multifrequency dial signals from different telephone circuits. Such multiplexing involves a further dilemma of conflicting design objectives. The complexity and expense of multiplexing equipment tends to increase with the frequency at which each of the input lines must be addressed. On the other hand, reduction of that sampling frequency tends to reduce correspondingly the clock frequency by which the tone cycles are timed, complicating reliable distinction between closely adjacent signal tones.
The present invention resolves that conflict. As a result of its novel frequency discriminating technique, the invention can operate effectively at a relatively low clock frequency, permitting a satisfactory number of input circuits to be sampled at a convenient and economical rate.